java-system-exit问题

发表于 更新于 阅读次数：

System.exit(1)有什么用

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public class SystemExitDemo {

    public static void main(String[] args) {

        System.out.println("begin...");

        doSometings();

        try {
            //主线程阻塞一会儿，让子线程先执行完
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("end...");
    }

    public static void doSometings() {

        Thread thread = new Thread() {

            @Override
            public void run() {
                System.out.println(111);
                System.exit(1);
            }
        };
        thread.start();
    }

}
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begin...
111

Process finished with exit code 1

从输出结果看，System.exit(status)会退出整个jvm，而不只是当前线程。

自定义SecurityManager

System.exit(status) ==》 Runtime.getRuntime().exit(status)

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public void exit(int status) {
     SecurityManager security = System.getSecurityManager();
     if (security != null) {
         security.checkExit(status);
     }
     Shutdown.exit(status);
 }

如果想要控制exit，需要自定义SecurityManager来处理 

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public class MySecurityManager extends SecurityManager {

    @Override
    public void checkExit(int status) {
//        super.checkExit(status);
        throw new SecurityException("not allow user System.exit");
    }
}


public class SystemExitDemo {

    public static void main(String[] args) {

        System.setSecurityManager(new MySecurityManager());
        System.out.println("begin...");

        try {
            doSometings();
        } catch (Exception e) {
            e.printStackTrace();
        }

        try {
            //主线程阻塞一会儿，让子线程先执行完
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("end...");

    }

    public static void doSometings() {

        Thread thread = new Thread() {

            @Override
            public void run() {
                System.out.println(111);
                System.exit(1);
            }
        };
        thread.start();
    }

}

输出：

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begin...
111
Exception in thread "Thread-0" java.lang.SecurityException: not allow user System.exit
	at com.mydemo.MySecurityManager.checkExit(MySecurityManager.java:8)
	at java.lang.Runtime.exit(Runtime.java:107)
	at java.lang.System.exit(System.java:962)
	at com.mydemo.SystemExitDemo$1.run(SystemExitDemo.java:32)
end...

总结

  1. System.exit(status)默认情况下是暴露给SecurityManager来控制并退出jvm的
  2. 只是在程序中达到退出线程的目的，应该使用异常而不是exit